McOsu is an open-source practice client for the PC rhythm game osu!, developed from scratch by McKay42. While it is playable without an osu! installation or account, it does not support osu!'s online features and ranking system and lacks some gameplay elements from the main game such as health.
McOsu allows the player to individually override the difficulty values of the map he is playing (circle size, approach rate, overall difficulty and even game speed/song BPM). It also includes a multitude of features that can help with practice; a target practice mod, mid-song pause skipping and scrubbing, an enhanced hit error bar, a powerful console and more. In addition, it includes many quality of life features (improved aesthetic customization, HUD scaling, changing game options including mods and value overrides while playing...) and many experimental features such as a 3D first-person mode, a VR mode, and various mods which can make notes wobble or flash in rainbow colors.
McOsu is developed separately from osu! and peppy is not involved with its development in any way.